A gas turbine engine contains a compressor in fluid communication with a combustion system that often contains a plurality of combustors. The compressor raises the pressure of the air passing through each stage of the compressor and directs it to the combustors where fuel is injected and mixed with the compressed air. The fuel and air mixture ignites and combusts creating a flow of hot gases that are then directed into the turbine. The hot gases drive the turbine, which in turn drives the compressor, and for electrical generation purposes, can also drive a generator.
Most combustion systems utilize a plurality of fuel injectors for staging, emissions purposes, and flame stability. Fuel injectors for applications such as gas turbine combustion engines direct pressurized fuel from a manifold to the one or more combustion chambers. Fuel injectors also function to prepare the fuel for mixing with air prior to combustion. Each fuel injector typically has an inlet fitting connected either directly or via tubing to the manifold, a tubular extension or stem connected at one end to the fitting, and one or more spray nozzles connected to the other end of the stem for directing the fuel into the combustion chamber. A fuel passage (e.g., a tube or cylindrical passage) extends through the stem to supply the fuel from the inlet fitting to the nozzle. Appropriate valves and/or flow dividers can be provided to direct and control the flow of fuel through the nozzle and/or fuel passage.
U.S. Pat. No. 6,718,770 to Laing et al. discloses a gas turbine fuel injector including a single feed strip (fuel passage) contained in a hollow stem of the injector. In one embodiment, the feed strip includes a curved middle portion with a radius of curvature greater than a length of the middle portion so that the strip can be easily inserted and withdrawn from the hollow stem without placing undue stress on the strip.
The fuel injectors are often placed in an evenly-spaced annular arrangement to dispense (spray) fuel in a uniform manner into a combustor. Additional concentric and/or series combustion chambers each require their own arrangements of nozzles that can be supported separately or on common stems. The fuel provided by the injectors is mixed with air and ignited, so that the expanding gases of combustion can, for example, move rapidly across and rotate turbine blades to power an aircraft.
Of particular concern in the design of any component of a gas turbine engine is high cycle fatigue. High cycle fatigue in turbine engines occurs when resonance or vibration modes of parts like fuel injectors, turbine blades, compressors, or rotors are excited by driving frequencies inherent in the operation of the engine. For example, shaft rotation imbalance can produce driving frequencies between about 200 to about 300 Hertz (Hz). Driving frequencies due to combustion rumble can be in the range of about 300 Hz to about 800 Hz. Fuel pump pulsations can produce driving frequencies in the range of 1200 Hz. Blade passing frequencies can be upwards of 1200 Hz.
Prior art fuel injectors have incorporated devices, such as the one shown in U.S. Pat. No. 6,038,862, to address the issue of high cycle fatigue. Typically, such devices are intended to damp vibration of the parts to avoid resonance. However, such devices can be complex and require additional parts which can resonate themselves. Another approach has been to alter the natural frequency, also referred to herein as resonant frequency, of the parts. In general, reinforcing ribs and/or additional structure is provided to increase the natural frequency of the part above the anticipated driving frequencies of the turbine.
Another approach has been to alter the natural frequency of the part by shaping the part such that its natural frequency is above the maximum driving frequency the part will experience. For example, U.S. Pat. No. 6,098,407 discloses a fuel injector including a fuel supply tube that is coiled into a 360 degree spiral shape. Ideally, the curvature of the tube is such that the tube's natural frequency is well above the maximum vibratory frequency that the tube will experience during engine operation.
The above-described approaches for dealing with high-cycle fatigue, although effective for many applications, tend to add bulk to the parts which can take up valuable space in and around the combustion chamber, block air flow to the combustor, and add weight to the engine. Additional structure also tends to increase the stiffness of the parts which can be undesirable in applications where flexibility of the part is desired or necessary. This can all be undesirable with current industry demands requiring reduced cost, smaller injector size, and reduced weight for more efficient operation.